1. Field of the Invention
The present invention relates to angular velocity sensors, and in particular, relates to a supporting structure of a tuning-fork resonator used in an angular velocity sensor.
2. Description of the Related Art
One known angular velocity sensor is disclosed in Japanese Unexamined Patent Application Publication No. H11-230758 (hereinafter referred to as Patent Document 1). This angular velocity sensor (not shown) has a tuning-fork resonator and a casing accommodating the tuning-fork resonator. The tuning-fork resonator includes a pair of parallel prongs arranged such that the prongs are connected together at one end thereof. The tuning-fork resonator is accommodated in the casing so that the connection portion of the prongs is fixed to a seat on a bottom surface of the casing and the connection portion is a non-vibrating portion (node). This tuning-fork resonator fundamentally undergoes vibration wherein the tips (open ends) of the prongs open and close during driving, i.e., vibration in which the open ends of the prongs move in together and out together. This vibration is referred to as parallel vibration.
When an angular velocity about a rotational axis in the longitudinal direction of the prongs, i.e., the longitudinal direction of the tuning-fork resonator, is applied to the tuning-fork resonator undergoing parallel vibration, another vibration in which each of the prongs vibrates oppositely in a direction that is perpendicular to principal surfaces of the tuning-fork resonator is generated by the Coriolis force. This vibration is referred to as asymmetrical out-of-plane vibration. The principal surfaces of the tuning-fork resonator are two surfaces defined by the longitudinal direction and lateral direction of the prongs, which are arranged parallel to each other.
For example, when a tip of a first prong moves in one direction (upward) that is perpendicular to the principal surfaces of the tuning-fork resonator, a tip of a second prong moves in another direction (downward). Therefore, detecting and comparing the direction and the amount of asymmetrical out-of-plane vibration in each of the prongs enables detection of the direction of rotation and angular velocity applied to the tuning-fork resonator.
In the angular velocity sensor disclosed in Patent Document 1, to reduce the profile of the angular velocity sensor, the principal surfaces of the tuning-fork resonator and the bottom surface of the casing are opposed to each other, and a connection portion of the tuning-fork resonator is directly fixed to a seat in the casing. In this structure, however, the connection portion of the tuning-fork resonator effectively functions as a non-vibrating portion in parallel vibration, but it does not effectively function in asymmetrical out-of-plane vibration generated by application of an angular velocity.
According to the investigation conducted by the inventors of the present invention and others, it has been established that, in the angular velocity sensor of Patent Document 1, the connection portion of the tuning-fork resonator to which an angular velocity is applied, especially both edges of the connection portion in the width direction, undergoes torsional vibration that is greater than the center thereof. Therefore, it has been determined that asymmetrical out-of-plane vibration generated by the application of an angular velocity to the tuning-fork resonator is poorly energy-trapped, and the asymmetrical out-of-plane vibration leaks from the connection portion of the tuning-fork resonator to the casing via the seat.
In this structure, even if the resonance frequency or detection accuracy of the tuning-fork resonator is adjusted in advance, the resonance frequency or detection accuracy of the angular velocity sensor varies from product to product and inevitably decreases after the casing accommodating the tuning-fork resonator is fixed to a circuit board. Due to insufficient blocking of vibration, energy leaks outside. This causes a poor Q of the tuning-fork resonator, thus failing to ensure a high sensitivity. As a result, the impedance during detection is disadvantageously reduced.
FIGS. 9 and 10 show a combined structure of a tuning-fork resonator and a supporting substrate included in an angular velocity sensor that can overcome the above-described disadvantages. FIG. 9 is an exploded perspective view of the combined structure. FIG. 10 is a perspective view of the combined structure, as seen from the back side.
Specifically, the angular velocity sensor has a bimorph tuning-fork resonator 51 in which base ends of a pair of parallel prongs 53 are fixed to a support 54. In the tuning-fork resonator 51, a pair of piezoelectric substrates are polarized oppositely in the thickness direction and are bonded together, a pair of driving external electrodes 57 are separately disposed in parallel on one surface of each of the prongs 53, which are divided by a slit 55, and common external electrodes 56 are disposed on the other surfaces of the prongs 53. The angular velocity sensor having the above-described structure is disclosed in Japanese Unexamined Patent Application Publication No. 2004-061486.
This angular velocity sensor further includes a supporting substrate 52 on which the tuning-fork resonator 51 is mounted and a casing (not shown) accommodating the tuning-fork resonator 51 by fixing the supporting substrate 52 to the casing. The tuning-fork resonator 51 is mounted on the supporting substrate 52 such that a principal surface of the supporting substrate 52 and a principal surface of the tuning-fork resonator 51 are opposed to each other. The supporting substrate 52 supports the prongs 53 at a support 54 acting as a base to which base ends of the prongs 53 are fixed so that the prongs 53 can vibrate. In this angular velocity sensor, after the tuning-fork resonator 51 is mounted on the supporting substrate 52 and then the resonance frequency and detection accuracy are adjusted in this combined structure, the tuning-fork resonator 51 is accommodated in the casing by fixing the supporting substrate 52 to the casing.
In this angular velocity sensor, it is possible that a non-vibrating portion of the supporting substrate 52 to which vibration would leak from the tuning-fork resonator 51 is identified in advance and the supporting substrate 52 is fixed to the casing at the identified non-vibrating portion. In this structure, even if asymmetrical out-of-plane vibration leaks from the tuning-fork resonator 51 to the supporting substrate 52, the vibration does not leak from the supporting substrate 52 to the casing. As a consequence, the various disadvantages described in Patent Document 1 would be solved.
However, according to the investigation conducted by the inventors of the present invention and others, it has been established that a non-vibrating portion S of the supporting substrate 52 of the angular velocity sensor is recognized only in the substantially central portion of the supporting substrate 52 in the longitudinal direction of the tuning-fork resonator 51 mounted thereon. Outer regions of the supporting substrate 52 vibrate so as to rotate about the substantially central portion of the supporting substrate 52 and undergo displacement, as shown in FIG. 10. Therefore, in the angular velocity sensor having this structure, the non-vibrating portion S of the supporting substrate 52 must be fixed.
The arrows illustrated in FIG. 10 represent the directions and the amounts of displacements of areas (corners) of the supporting substrate 52 at a certain point in time during operation. As is apparent from FIG. 10, the supporting substrate 52 vibrates so as to rotate about the non-vibrating portion S.
However, fixing the non-vibrating portion S recognized in the substantially central portion of the supporting substrate 52 while isolating the outer regions of the supporting substrate 52 may cause the fixing portion to interfere with mounting of other necessary units, such as a circuit unit, or to prevent miniaturization of the angular velocity sensor. In other words, since the supporting substrate 52 does not have a non-vibrating portion S in the outer region, the supporting substrate 52 cannot be fixed to the casing at the outer region.